1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic system and, more particularly, to an image forming apparatus having a discriminating unit for discriminating the mounting/ dismounting of a developing unit used in the apparatus, the presence/absence of a developer in the developing unit, and the toner density of the developer. 2. Description of the Related Art
A development system used in an image forming apparatus such as a copying machine or various types of printers includes a developing unit arranged near the outer surface of a photosensitive drum. A developing roll is arranged in the developing unit and a developer attached to the developing roll is positioned to be brought into slidable contact with the photosensitive drum. A toner which is sufficiently agitated by an agitating rod is attracted to a peripheral portion of the developing roll. A toner sensor for detecting a toner residual amount is mounted in the lower portion of the developing unit. A piezoelectric type sensor and a reactance detection type sensor are known as toner sensors. When the lack of the developer is detected by the sensor, an operator is informed of the lack, and a new developer is replenished.
FIG. 1 shows a state wherein a toner sensor 7 is electrically connected to a printer controller 8 in a printer main body through cables. Referring to FIG. 1, reference numeral 9 denotes a toner detector for supplying a signal from the toner sensor 7 to a CPU 10. The CPU 10 incorporates a ROM for storing a control program, a RAM for temporarily storing data to be processed by the CPU 10, and the like, and constitutes a central processing unit. Reference numeral 11 denotes a display section for displaying the presence/absence of a toner and error contents.
An output from the toner sensor 7 for detecting the presence/absence of a toner in the developing unit 1 is supplied to the toner detector 9. Upon detection of the lack of a toner, the toner sensor 7 outputs, e.g., a 3.5-kHz rectangular signal. This signal is smoothed by resistors R1 to R3 and capacitors C1 and C2 in the toner detector 9, and is supplied to a transistor Q1 as a high-level signal so as to turn on the transistor Q1 and a transistor Q2. As a result, a low-level signal, as a signal representing the lack of the toner, is input to a terminal IN1 of the CPU 10. If the toner sensor 7 detects the presence of a toner, the toner sensor 7 stops outputting the rectangular signal. Consequently, the transistors Q1 and Q2 are turned off, and a high-level signal is output to the terminal IN1 of the CPU 10. The CPU 10 causes the display section 11 to display the presence/absence of a toner in accordance with the low- or high-level signal input from the terminal IN1.
Since the developing unit 1 is designed to be detachably mounted, as a unit, in the printer main body, it can be exchanged with another developing unit when a developer having a different color is to be used or the service life of a developer comes to an end. However, the printer main body must detect whether a developing unit is mounted or not, and inhibit a developing (printing) operation when no developing unit is mounted. In a conventional apparatus, the mounting/ dismounting of the developing unit 1 is detected in the following manner. When the developing unit 1 is not mounted in the printer main body, a high-level signal is input to an input terminal IN2 of the CPU 10 through a pull-up resistor R4. In contrast to this, when terminals 3' and 4', which are connected to each other on the developing unit 1 side, are respectively connected to input terminals 3 and 4 of the printer controller 8, i.e., the developing unit 1 is mounted in the printer main body, a low-level signal is input to the CPU 10. The mounting/dismounting of the developing unit 1 can be discriminated by using signal inputs which have different levels depending on the mounting/dismounting of the developing unit 1.
In the above-described image forming apparatus, however, extra signal lines are used only for the detection of the mounting state of a developing unit. In addition, when a toner density is to be detected, another extra signal line is required. This increases a rate of occurrence of noise. In addition, extra input ports of the CPU are required.
These problems lead to not only degradation in reliability of the apparatus but also an increase in manufacturing cost.